The scrubbing of flue gases with the aid of an annular-gap washer is known, for example, from commonly owned U.S. Pat. Nos. 3,726,065 and 3,791,108. These washers comprise a tubular body which is interposed between coaxial upper and lower parts of a descending gas-discharge duct to form a constriction or waist, that body coacting with an insert which can be raised or lowered to vary the effective width of an annular gap formed between them. The vertical adjustment of the insert controls the pressure differential existing across the gap as well as the flow velocity at its exit end.
In eariler systems of this type, as illustrated in the above-identified patents, the insert is generally pear-shaped and defines the controlled gas passage with an upper portion of the surrounding nozzle body above its waist. In commonly owned application Ser. No. 670,542, filed by me jointly with others on Mar. 25, 1976, there has been disclosed a frustoconical insert whose downwardly diverging peripheral surface defines an annular gap with the lower portion of the nozzle body. The top of the frustocone projects into the duct above the waist and defines a downwardly converging throat with the upper portion of the nozzle body.
In contrast to the earlier pear-shaped inserts, whose downwardly converging flow-controlling surfaces were sheltered from direct, impingement by the oncoming gas, the downwardly diverging inserts are directly exposed to the gas in the upper part of their peripheral surface which is therefore subject to rapid wear and corrosion.